The invention relates to a method for welding polyolefinic surfaces in the presence of heat, while exerting pressure, whereby the polyolefinic surfaces are heated to their melting temperature.
Such a method is commonly known. According to this method polyolefinic surfaces to be connected of objects of the same polyolefinic material, particular pipes, are heated to above their melting point, whereupon the heated ends are pressed against each other a welded joint, thus being formed.
This known method presents the disadvantage that the formed joint does not suffice when object surfaces consisting of polyolefins with a different melt-flow-index should be welded. For melting the polyolefinic surfaces to be interconnected, a heating element with a uniform temperature is used so far, to be utilized for heating both surfaces, so that when surfaces of polyolefins with a different melt-flow-index are to be welded the two surfaces have a wholly different flow behaviour.
Polyolefins with different properties can be defined by their melt-flow-index, which is determined by measuring the quantity of liquid polymer put in grams, which per 10 minutes flows through a mouth piece of particular dimensions under a particular pressure and at a particular temperature. When the pressure is increased the velocity increases but there is no rectilinear connection between the quantity of flown out polymer per 10 minutes and the pressure.
In view of the complex connection between temperature, viscosity, melt-flow-index and pressure it is impossible to obtain a proper joint when performing the known methods when surfaces of polyolefinic objects with a different melt-flow-index should be welded.